Why Is Welding Aluminum Difficult?
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Before highlighting different welding processes that are used for joining aluminum, it is important to understand some of the difficulties that are inherent to welding aluminum. One area of difficulty is filler metal. First, some aluminum alloys cannot be welded without filler materials. Alloys such as 6061 will undergo solidification cracking if welded without filler metal. Furthermore, the correct filler material must be selected. For instance, welding a 6061 alloy with a 6061 filler metal will result in weld failure. Instead, a 5356 or 4043 aluminum filler metal should be used when welding a 6061 base material. Another challenge with aluminum filler metal is feeding. If a mechanical wire feeding process is being used, special drive systems will most likely be needed. This is because aluminum has less column strength than steel, and will more than likely buckle and tangle if special wire drive systems, such as a push-pull gun, are not used. This is especially true for thinner aluminum filler metals (i.e. 0.8 mm or 1 mm diameter).
Aluminum also has a greater thermal conductivity than steel. The heat created when the welding process is initiated on aluminum is dispersed more rapidly than when welding an iron-based alloy. Therefore, full penetration may not occur until the weld has progressed quite far from the start. This is known as a cold start. Care must be taken so that cold starts do not occur when welding aluminum. Another result of the increased thermal conductivity is larger craters. By the time the end of the weld is reached, more heat is present than at the start. This heat disperses well in aluminum and can create a large crater. Aluminum is very susceptible to crater cracking, therefore, craters should be filled in so that failure does not occur at the end of a weld.
Aluminum also requires different pre-weld and post-weld processing. Aluminum forms an oxide layer that has a higher melting temperature than the actual aluminum itself. In order to avoid un-melted aluminum oxide particles in the weld, an oxide removal process, such as wire brushing or chemical cleaning, should be used prior to welding. Several aluminum alloys, such as 6061-T6, are artificially aged to increase their strength. The heat from welding ruins the benefits gained by artificial aging, and large reductions in strength will be found in the heat-affected zone. Therefore, post-weld artificial aging may be required for alloys such as these.